Reduction of adjacent channel interference

ABSTRACT

An improved system and method for reducing adjacent channel interference radio systems by making use of the so called &#34;cocktail party effect&#34;. Also, the selectivity is improved by measuring the level of adjacent channel interference above and below the desired channel and automatically changing selectivity characteristics of the receiver in accordance with the measured interference. 
     The system is particularly applicable to the reception of AM Stereo signals but it may also be used to improve the performance of monophonic systems.

This is a continuation of application Ser. No. 764,476 filed Jan. 31,1977 now abandoned.

BACKGROUND OF THE INVENTION

While the invention is subject to a wide range of applications, it isespecially suited for use in reducing the effects of adjacent channelinterference to AM Monophonic and AM Stereo systems and will beparticularly described in those connections.

There have been many procedures used in the past for reducing the effectof adjacent channel interference, including variable selectivity filterswhich are manually or automatically operated and audio or IF notchfilters which are manually adjusted to sharply discriminate againstinterference. See, for example, page 517, 518 and 543, 544 K. P.Sturley, "Radio Receiver Design", second edition 1953, John Wiley &Sons, Inc., New York.

Also, there have been many types of systems developed for automaticallyaltering the selectively of receivers as a function of interference. Theselectivity may be varied symmetrically or asymmetrically. Discussion ofsuch systems is provided by Sturley on pages 543 and 544 of his bookcited above. The variation in selectively is provided at intermediatefrequency as is the sensing system.

The object of the present invention is to provide inexpensive automaticmeans for reducing interference while maintaining relatively goodfrequency response.

A further object is to provide a method for reducing adjacent channelinterference which is especially suitable for AM Stereo reception.

The invention also makes use of the so-called "cocktail party effect" tofurther discriminate against interference. The cocktail party effect isthat effect that allows a binaural listener to separate speech arrivingfrom two different talkers and pay more attention to one of the talkers.

A major object of the invention is to allow listeners of AM monophonicand stereo signals to take advantage of the "cocktail party effect".

SUMMARY OF THE INVENTION

The present invention may be used in the reception of amplitudemodulated waves, and is especially suitable for the reception of certaintypes of AM Stereo waves. The invention may, for example, be used toreduce adjacent channel interference in the reception of independentsideband (ISB) types of AM Stereo modulated waves by;

(a) measuring the level of the received interference in the uppersideband stereo channel caused by the interfering channel contiguous tothe upper sideband of the desired signal,

(b) measuring the level of the received interference in the lowersideband stereo channel caused by the interfering channel contiguous tothe lower sideband of the desired signal, and

(c) automatically reducing the audio frequency response of the stereochannel suffering from the greater interference level according towhether (a) or (b) measurement is greater, when the level of thestronger interference level exceeds a predetermined level.

In many applications of the invention it is desirable to attenuate boththe adjacent channel above and below the desired channel by a largerfactor than when the system is operating free of substantialinterference. However, it is desirable to provide greater attenuationfor the adjacent channel suffering from the larger interference level.

The invention may also be used to improve the performance of monophonicreceiving systems by allowing the listener to enjoy the use of thecocktail party effect. One method for improving the adjacent channelinterference performance of receivers of monophonic double-sidebandsignals includes the steps of isolating and demodulating both the upperand lower sideband of the desired signal and feeding the resulting audiowave to circuitry suitable for driving separate transducers. Thetransducers can be loudspeakers set up spacially as one would set upstereo loudspeakers; i.e., spaced some 4 to 6 feet apart. It is alsopossible to practice the invention by using stereo headphones as thetransducers.

There are a number of ways of isolating and demodulating the upper andlower sideband channels, including phase shift type systems and filtertype systems. Such techniques are well known to those skilled in theart. The system may be used with full carrier or double-sideband reducedcarrier or suppressed carrier waves; i.e., with full carrier waves orwaves wherein the carrier level is less than the peak combined amplitudeof the sidebands.

The monophonic application of the invention can also utilize means formeasuring the level of the interference in the channels contiguous tothe upper and lower sideband components of the desired signal andreducing the audio frequency response of the channel suffering from thegreater interference level. Thus, the listener will not only be allowedto enjoy the improved interference reduction effects of the cocktailparty effect but will also enjoy the improved selectivity of the system.

This invention is especially useful for receivers of independentsideband (ISB) type stereophonic signals where the stereo relatedinformation appears on the upper and lower sideband modulation of thecarrier wave. In one such type of signal, the carrier is amplitudemodulated by the stereo summation, L+R, intelligence and simultaneouslyphase modulated with the stereo difference, L-R, intelligence. Such astereo wave is described in detail in U.S. Pat. Nos. 3,218,393 and3,908,090 for example. The receivers for such a stereo wave can bedesigned to accentuate the cocktail party by providing specialcharacteristics of the phase shift sideband separation device whichwould normally operate at intermediate frequencies and would be fed by asuperheterodyne type circuit. The phase shift separation device, unlikea normal stereo receiver, would provide substantial separation foradjacent channel interference signal components falling above 5 kHzwhich is normally the acceptable high frequency range of stereoseparated components. By connecting the output of the phase shiftsideband separation device to means for separately amplifying the audiooutputs of the sideband separation device, the interference componentscan be caused to fall at one stereo loudspeaker and whereas the adjacentchannel interference to the other sideband would fall at the secondstereo speaker. The desired stereo components would generally fallinbetween the two speakers as is normal for the stereo illusion.

In both stereophonic and monophonic reception, the reduction in audiofrequency response should only be applied when the worst sidebandchannel interference is of sufficient amplitude to create listeningproblems. Generally, if the power level of the interference is less thanone-tenth of a percent of the desired sideband power level it isunnecessary to reduce the audio frequency response. Typically, theresponse of the channel suffering from a stronger interference level isreduced to approximately one-third to the full response of the channel.For example, in medium quality stereo reception a 6 kHz audio responsewould be reduced to 2 kHz. For monophonic communications applications,the response would be reduced from 3 kHz to 1 kHz for example. The sidesuffering from less interference would be reduced by a lesser degree andfor some applications of this invention the side receiving the weakerinterference would not be altered in frequency response.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription, taken in connection with the accompanying drawings, whileits scope will be pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a typical receiving system constructed inaccordance with the teachings of the present invention.

FIG. 2 is a schematic/block diagram of a switching arrangement forswitching the lowpass filters into the upper and lower sideband channelsto provide improved interference performance.

FIG. 3 is a sketch showing severe interference as one might see on aspectrum display device. An object of the present invention is toimprove reception performance under this type of interference.

FIG. 4 shows a prior art phase shift form of independent sideband AMstereo receiver as disclosed in FIG. 11 of U.S. Pat. No. 3,218,393.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a block diagram of the receiver embodying the invention. Thistype of receiver may be used for the reception of certain forms of AMStereo signals; i.e., independent sideband type (ISB) stereo signalswherein essentially the left band stereo channel information istransmitted via one sideband and the right band stereo channelinformation is transmitted via the other sideband. The circuit shown mayalso be used for the reception of monophonic signals. When used in thereception of monophonic signals, it, besides providing improvedselectivity, has the added advantage over conventional AM monophonicreception of spatially separating interference from the desired signal.

The desired monophonic signal and many of the components of a stereosignal will appear to come from a point mid-way between the left andright loudspeakers. However, adjacent channel interference above thedesired channel will appear to come from a point to the left of thedesired signal and interference below the desired signal will appear tocome from a point to the right side of the desired signal.

As mentioned above, this allows the listener to discriminate against theinterference and favor the desired signal. To understand how the systemprovides improved selectivity it is necessary to examine FIG. 1.

The antenna 102 is connected to the RF-IF superheterodyne circuit 104.The IF output of 104 feeds amplifier 106. The output of 106 whichappears on line 107 feeds three separate circuits; an upper sideband, alower sideband, and a carrier channel circuit. The upper circuit isisolated by filter 108 which in turn feed; amplifier 114 which feedsproduct demodulator 118. The injection port of the product demodulator118 is connected to the carrier channel 110 by line 111. The carrierchannel is used to select the carrier signal and provide a clean carrierwave. One circuit which may be adapted for such service is described inU.S. Pat. No. 3,973,203.

There are a number of methods for isolating and demodulating theintelligence in the upper and lower sideband of a double-sideband waveincluding the phase shift system in addition to the system shown inFIG. 1. An improved method of receiving ISB AM Stereo signals isdescribe in detail in patent application Ser. No. 573,905, now U.S. Pat.No. 4,018,994 which issued Apr. 19, 1977.

The lower sideband is selected by filter 112 which in turn feedsamplifier 116 which then feeds product demodulator 120. Productdemodulator 120 is also fed by the output of carrier channel 110 throughline 111. In one version of this system, product demodulators 118 and120 may be replaced by envelope demodulators. Such an arrangement wouldalso allow the elimination of carrier channel 110. However, for minimumdistortion, and best signal-to-noise operation in fringe area locations,the circuit utilizing product demodulators as shown in FIG. 1 issuperior. Also, if envelope demodulators are used without the carrierchannel circuit, the upper and lower sideband filters would be requiredto pass the carrier component complicating their specifications.

The output of product demodulator 118 feeds amplifier 122 and productdemodulator 120 feeds amplifier 124. The output of amplifier 122 feedshighpass filter 138 which in turn feeds detector 140. Similarly, theoutput of amplifier 124 feeds highpass filter 142 which in turn feedsdetector 144. The filters 138 and 142 and the detectors 140 and 144 areused to measure the level of the interference contiguous to the desiredsidebands.

In present broadcast service, the carrier frequencies on the standardbroadcast AM band in the United States are separated by 10 kHz. Filters138 and 142 are used to isolate the co-channel interference signals.Therefore, filters 138 and 142 should pass 10 kHz waves with relativelylittle attenuation.

In order to conveniently sense the level of the adjacent channel carrierit is necessary that the filters in the RF and IF circuits 104 and thesideband filters 108 and 112 be sufficiently wide to pass signalsdisplaced from the center or desired carrier frequency by at least ±10kHz.

The output of detectors 140 and 144 are connected to comparison circuit146 through lines 141 and 145. Comparison circuit 146 evaluates which ofthe detectors is producing a higher level wave. Thus, if detector 140produces a larger output than detector 144 it can be assumed thatinterference to the upper sideband was greater than interference to thelower sideband. Comparison circuit 146 then, for example, would switchin a lower cutoff frequency adjustment of switcheable lowpass filter126. Conversely, if the output from detector 144 as appearing on line145, produces a greater voltage than the voltage appearing on 141, itwould be assumed that the interference to the lower sideband channel wasgreater and the switcheable lowpass filter 128 would switch to a lowercutoff frequency than filter 126.

The cutoff frequencies for filters 126 and 128 could be made a functionof the interference level or it could jump in discrete steps. Forexample, the channel suffering from the large interference level mightbe set for a cutoff frequency of one-third of its normal full bandpasswhich would be used when the channel is receiving a signal which isrelatively free of interference. For communications service, the cutofffrequency might be 1 kHz. The wideband filter which measures loweramounts of interference may, in one embodiment of the invention be leftat its full wideband characteristic or it might be reduced in bandwidthbut by a lesser amount than the sideband which suffers from the strongerinterference.

The reason it would be advantageous in certain situations to reduce thefrequency response of both sidebands is that if either sideband issuffering from interference it generally indicates that the desiredsignal is weak. Thus, reducing its bandwidth may help combat poor noiseconditions.

The output of the switcheable or controllable lowpass filters 126 and128 feed individual amplifiers 130 and 132 which in turn feed, forexample, the left hand transducer fed by the upper sideband and theright hand transducer fed by the lower sideband.

In typical applications of this equipment, the transducers would beconventional loudspeakers although stereo headphones may be used in manycases.

It should be noted that while FIG. 1 illustrates the use of the filtermethod of isolating sidebands, the invention is also applicable to thephase shift method of sideband selection. Phase shift type receivers arewell known in the art and one technique based upon phase shift typesideband reception is disclosed in patent application Ser. No. 573,905,now U.S. Pat. No. 4,018,994 which issued Apr. 19, 1977. In that patentapplication a circuit especially suitable for compatible AM Stereoreception is shown. The circuit can be altered to provide the advantagesof the new invention by taking the output as shown in that FIG. 1(application Ser. No. 573,905, now U.S. Pat. No. 4,018,994 which issuedApr. 19, 1977) before the stereo signals are fed to loudspeaker 68 and70 and feeding them to the circuitry shown in FIG. 1 of thisspecification connected to the outputs of 122 and 124. Of course all ofthe circuitry shown in FIG. 1 of the instant application prior to thispoint in the system is no longer required; i.e., all of the circuitryincluding amplifiers 122, 124 and the blocks feeding these amplifierswould be deleted as the stereo separation circuitry in FIG. 1 ofapplication Ser. No. 573,905, now U.S. Pat. No. 4,018,994 which issuedApr. 19, 1977, performs all of the necessary signal processing which isperformed by the deleted blocks. It should be noted that the phase shiftnetworks 68 and 60 of FIG. 1 of patent application Ser. No. 573,905, nowU.S. Pat. No. 4,018,994 which issued Apr. 19, 1977, must be altered indesign so that at least a small amount of sideband suppression isprovided at 10 kHz. While separation at this higher frequency isgenerally unnecessary for stereo reception it is necessary for sensingthe adjacent channel interference in the instant invention. Separationof higher frequencies is also necessary if the cocktail party effect isto be fully enjoyed.

As indicated above, conventional phase shift type SSB receivers asdescribed in the literature may also be used with this invention formonophonic reception.

If the sideband isolated by filter 108 and demodulated by block 118represents the upper sideband and the interference above the desiredchannel is greater than the interference below, signal filter 126 willbe switched to a lower frequency cutoff, than filter 128 and theinterference will be attenuated. It is also possible to continuouslyreduce the high frequency cutoff of the lowpass filter as a function ofthe interference rather than using a switcheable filter as shown in FIG.1 below.

In another embodiment of the invention, both filters 126 and 128 arecontrolled by reducing their cutoff frequencies but the channelsuffering from the greater amount of interference is provided with thelowest cutoff frequency.

Instead of utilizing highpass filters connected at the output to theoutput of amplifiers 122 and 124 bandpass filters connected to theoutput of amplifier 106; i.e., in line 107 can be used. However, suchfilters, if they are to provide reasonable selectivity, would besomewhat more expensive than the filters shown in block form in FIG. 1;i.e., highpass filters 138 and 142.

The outputs of switcheable lowpass filters 126 and 128 feed amplifiers130 and 132 which feed transducer circuits. The transducers may beloudspeakers or stereo type headsets. When properly located loudspeakers(for example, in home installations four to six feet apart and inautomobiles or other confined locations the speakers may be placedcloser together) or when stereo type earphones are used in the receptionof monophonic double-sideband signals with identical information intheir upper and lower sidebands the "cocktail party effect" can beutilized to discriminate against adjacent channel interference. As anexample, if the adjacent channel interference below the desired channel;i.e., the channel contiguous to the lower sideband of the desiredsignal, appears to the left of the listener, the adjacent channelinterference above the desired channel; i.e., the channel contiguous tothe upper sideband of the desired signal, appears to the right of thelistener. The desired upper and lower sideband intelligence is fed inphase to the speaker producing an illusion of the desired signal comingfrom the center; i.e., half way between the speakers.

By this procedure, the listener is allowed to enjoy the so called"cocktail party effect" and discriminate against the interfering signalswhich will appear to the side of the listener.

The same effect is present to some degree in the above described AMStereo embodiment of this invention. (Except the lower sidebandinterference would fall to the right of the listener and the upper tothe left.) However, since the desired signal components do not all fallin the center but may appear at locations closer to the speakers, alesser cocktail party effect advantage will be provided.

FIG. 3 shows a typical spectrum situation for a standard AM broadcastsystem wherein the desired carrier frequency is designated F_(c). Theadjacent channel 10 kHz above the desired carrier is designated at F_(c)+1. The 20 kHz upper adjacent channel carrier falls at F_(c) +2.Similarly, the lower frequency adjacent channels interfering signalshave carrier frequencies at F_(c) -1 and F_(c) -2.

It should be noted that there is appreciable sideband overlap becausemost quality AM broadcast stations have sideband components appreciablyhigher than 5 kHz. The sideband components from the co-channel stationstherefore can fall within the passband region of the desired signal.When they do, they can create a strange type of interference sometimescalled "monkey chatter" which is unintelligible and quite annoying tolisten to.

Thus, even if a receiver completely eliminates the carrier component ofthe adjacent channel interference, it will suffer from interference fromthe sidebands of the adjacent channel signals. The present invention, byappreciably attenuating signals closer than 10 kc from the desiredcarrier not only helps remove the 10 kHz heterodyne whistle but alsogreatly attenuate the sideband interference or monkey chatter. In theexample shown, in FIG. 3, the interference from the adjacent channelinterfering signal above the carrier frequency is appreciably strongerthan the interference from the lower sideband interfering wave.Therefore, it is important that the frequency response of the receiverbe substantially reduced for its upper-sideband components. If thereceiver was located at a point where its lower sideband would sufferfrom the higher amounts of interference it would be necessary to switchin additional selectivity for the lower sideband.

It is generally true that the frequency assignments are made so that ina given area there is very little interference from adjacent channelsignals spaced by 10 or 20 kHz from a given assigned station. However,if a listener tunes into a distant station there is a strong possibilitythat it will suffer from adjacent channel interference.

As pointed out above, the invention utilizes the so called cocktailparty effect to enhance the adjacent channel interference rejectionproperties of receivers.

The realization of the cocktail party effect requires that the apparentlocation of the desired sound source be spatially separated from theapparent source of the interfering waves. Thus, it is important that thesideband separation for both the AM Stereo equipment of this invention,and the monophonic embodiment be effective for all components fallingwith the effective high frequency limit of the audio passband. However,it is not as important that the separation be large for the lowfrequency section of the audio passband as adjacent channel interferencecomponents generally will not fall close to the desired signal'scarrier. Therefore, in the monophonic embodiment, the circuitryproviding separation of the sidebands need not be particularlyeffective, for, say, frequencies below 1 or 2 kHz. Thus, if the phaseshift type system is used for separating the upper and lower sidebands,the networks do not have to accurately provide phase correction at thelow frequency range and, accordingly, less components are required.

In the case of AM Stereo, the separation is required for both stereoperformance and for interference reduction. Therefore, the networks usedfor AM Stereo would require reasonable separation; say, down to at least300 Hz.

Furthermore, in the development of the AM Stereo system, as described inU.S. Pat. Nos. 3,218,393, 3,908,090, 3,947,749 and application Ser. No.573,905, now U.S. Pat. No. 4,018,994 which issued Apr. 19, 1977, thephase shift networks utilized in the receiver provided reasonably goodseparation; i.e., 20 db or more from 200 Hz to 5,000 Hz. However, theoverall monophonic frequency response of the receiver was somewhatgreater; i.e., up to 10 kHz. The reason circuitry providing separationfor the higher frequencies was not provided was that there is littlestereo information provided by the high frequency sounds if sufficientinformation is located in the low and middle range for one to identifylocation. Therefore, because of economic and other reasons, theseparation was limited to approximately 200 to 5,000 as above mentionedwhich approximately matched the separation characteristics of the stereotransmitter.

In the instant case, it is desirable to provide relatively goodseparation for the full high frequency response of the system so thatthe interference can be isolated and full use may be made of thecocktail party effect. However, the separation need not be very greatfor the listener to be able to discriminate against the interference.Generally, a 10 db separation figure is adequate.

Furthermore, if the embodiment of the invention incorporating additionalfiltering of the adjacent channel interference is utilized, theseparation need only be sufficient for the system to measure the levelof the interference wave. For example, where the equipment is used insystems where the carrier frequency of the adjacent channel interferenceis known, such as in standard AM broadcasting; i.e., 10 kHz from thedesired carrier, it is only necessary that the separation be reasonableat 10 kHz. Thus, it is unnecessary to provide the full cocktail partyeffect for those frequency components which, if sufficiently strong tobe disturbing, will be eliminated or at least greatly attenuated by thelowpass filtering as provided in FIG. 1 or FIG. 3. Actually, the networkcan be constructed so that the separation is poor, say above a fewkilohertz, as long as it is restored at 10 kHz. This easing ofspecification can substantially reduce cost and complexity of the meansfor providing separation of the upper and lower sideband components.

For details as to the construction of phase shift networks see"Normalized Design of 90° Phase-Difference Networks" by S. D. Bedrosianin "IRE Transactions of the Professional Group on Circuit Theory", Vol.CP-7, No. 2, at pages 128-136 (June, 1960) and the biographicalreferences therein.

FIG. 2 shows a switching arrangement which may be used to provide thedesired improved selectivity effect. Electronic switch 200, which iscomposed of Sections 202, 206, 208 and 212 is used to change thefrequency response of the system from full response (position 2) toreduce the upper sideband response to 2 kHz (position 1) and to reducefrom full response (position 2) the lower sideband to 2 kHz (position3). The switching circuit, if adapted to FIG. 1, would be controlled bycomparison circuit 146 and would be used in lieu of filters 126 and 128.Switch section 208 and section 212 are used to change the frequencyresponse of the system to the lower sideband intelligence. All theswitch sections are "ganged" or connected so that they change positionsin synchronism. In the position illustrated in FIG. 2 the upper sidebandis restricted in frequency response to 2 kHz by lowpass filter 204 andthe lower sideband to 3 kHz by lowpass filter 210. If the switches arecontrolled to the 3 position, as would be the case for severeinterference to the lower sideband, the lower sideband is limited to 2kHz response and the upper sideband to 3 kHz.

When interference is absent or low in level the switches would becontrolled to operate in the 2 position switching the filters out ofoperation and providing maximum frequency response.

In some systems it may be desirable to switch in only one filter andallow the other sideband to operate will full response. Such a switchingarrangement may readily be provided by altering the circuit of FIG. 2 byremoving filter 210 and connecting the lead formerly going to the inputof filter 210 to the lead formerly going to the output of filter 210.

It will be apparent to those skilled in the art that the audio filters204 and 210 can be placed at a number of other points or that acombination of filters may be used to achieve the desired improvedselectivity. The characteristics of the filter may be changed indiscrete steps or may be continuous. As will be apparent to thoseskilled in the art, the filters may be of the passive type utilizingdesign procedures described in many texts; such as, Filter Design Datafor Communication Engineers, J. H. Mole, John Wiley & Sons, Inc., NewYork, 1952. Also active audio filters may be used. The design proceduresfor active filters is shown in numerous texts; for example, RapidPractical Designs of Active Filters, by D. E. Johnson and J. L. Hilburn,published by John Wiley & Sons, Inc., New York, 1975.

In all cases, it is understood that the above described arrangements aremerely illustrative of the many possible specific embodiments whichpresent applications of the present invention. Numerous and other variedarrangements can be readily devised in accordance with the principles ofthe present invention without departing from the spirit and scope of theinvention.

What is claimed is:
 1. The method of reducing adjacent channelinterference in the reception of an independent sideband type AM stereosignal, comprising:(a) measuring the amplitude of a selected portion ofthe demodulated upper sideband of said received stereo signal that isrepresentative of the level of adjacent channel interference in saidupper sideband; (b) measuring the amplitude of a selected portion of thedemodulated lower sideband of said received stereo signal that isrepresentative of the level of adjacent channel interference in saidsideband; (c) comparing said measurements to develop an indication ofwhich sideband has the greater level of adjacent channel interference;and (d) in response to said indication, automatically reducing the audiofrequency response of the stereo channel corresponding to the sidebandhaving the greater level of adjacent channel interference.
 2. The methodof claim 1 including the additional step of automatically reducing theaudio frequency response of the stereo channel corresponding to thesideband having the lesser level of adjacent channel interference, butby an amount which is less than the amount by which the response of theother stereo channel is reduced.
 3. The method of claim 2 wherein thefrequency response of the stereo channel corresponding to the sidebandhaving the greater level of adjacent channel interference is reduced toapproximately 2 kHz and wherein the response of the other stereo channelis reduced to approximately 3 kHz.
 4. The method of claim 1 wherein theaudio frequency response of the stereo channel corresponding to thesideband having the greater level of adjacent channel interference isautomatically reduced only when the measurement corresponding to saidchannel exceeds a predetermined threshold level.
 5. The method of claim4 wherein said predetermined threshold level is approximately 0.1% ofthe corresponding sideband power level.
 6. The method of claim 1 whereinthe audio frequency response of the stereo channel corresponding to thesideband having the greater level of adjacent channel interference isreduced to approximately one-third the normal response of that stereochannel.
 7. The method of claim 1 wherein the audio frequency responseof the stereo channel corresponding to the sideband having the greaterlevel of adjacent channel interference is reduced to approximately 2kHz.
 8. An improved independent sideband (ISB) AM stereo signal receiverwherein the adverse effects of adjacent channel interference arereduced, comprising:(a) means for receiving a radio frequency ISB AMstereo signal and for converting the received signal to an intermediatefrequency (IF) signal; (b) means, responsive to said IF signal, fordemodulating said signal to develop a pair of stereo signalsrepresentative of left and right stereo information, respectively; (c) apair of audio frequency stereo signal translating channels, each fortranslating a corresponding one of said stereo signals; (d) means,responsive to a selected portion of a first one of said stereo signals,for developing a first indicating signal representative of the amplitudeof said portion and indicative of the amount of adjacent channelinterference in said first stereo signal; (e) means, responsive to aselected portion of a second one of said stereo signals, for developinga second indicating signal representative of the amplitude of saidportion and indicative of the amount of adjacent channel interference insaid second stereo signal; and (f) means, coupled to said stereo signaltranslating channels and responsive to said first and second indicatingsignals, for reducing the audio frequency response of the channel havingthe greater level of adjacent channel interference.
 9. An improved ISBAM stereo signal receiver in accordance with claim 8 wherein said lastmeans also automatically reduces the audio frequency response of thestereo signal translating channel having the lesser level of adjacentchannel interference, but by an amount which is less than the amount bywhich the response of the other channel is reduced.
 10. An improved ISBAM stereo signal receiver in accordance with claim 8 wherein said lastmeans reduces the audio frequency response of said channel only when thecorresponding indicating signal exceeds a predetermined threshold. 11.An improved ISB AM stereo signal receiver in accordance with claim 8wherein said last means reduces the audio frequency response of saidchannel to approximately one-third the normal response of said channel.12. A phase shift form of stereo receiver for independent sideband AMstereo signals having a stereo difference signal component of which hasbeen restricted to a predetermined upper stereo frequency limit,comprising:(a) means for receiving an RF AM stereo signal of the typedescribed hereinabove and for converting the received RF signal to anintermediate frequency (IF) signal; (b) means responsive to said IFsignal, for demodulating said signal to develop a pair of stereo sum anddifference signals; (c) means for shifting the relative phase of saidstereo sum and difference signals to develop a pair of phase-shiftedstereo sum and difference signals having a substantially quadraturephase relationship for audio frequencies lying within a predeterminedfrequency band extending above said upper stereo frequency limit; and(d) means, responsive to said phase shifted stereo sum and differencesignals for combining said signals to develop a pair of output stereosignals suitable for application to separate stereo audio frequencyamplifying and reproducing apparatus whereby the adverse effects ofadjacent channel interference will be reduced by the "cocktail partyeffect".
 13. A phase shift form of stereo receiver in accordance withclaim 12 wherein said means for shifting the relative phase of saidstereo sum and difference signals develops a pair of phase-shiftedstereo sum and difference signals having a substantially quadraturephase relationship for audio frequencies lying with a predeterminedfrequency band extending up to at least 10 kHz.